Time domain low field H-NMR analysis determines the quantity of hydrogen in a sample. In a typical time domain low field H-NMR analysis, a sample is inserted into a H-NMR instrument and subjected to a static polarizing magnetic field and to one or more radio frequency (RF) fields that are generated by a RF coil. The period of RF excitation typically lasts several microseconds and is known as the RF pulse. Following the RF pulse, a H-NMR signal is acquired. The H-NMR signal is generated in the RF coil. The hydrogen content of the sample is determined by comparing the intensity of the acquired H-NMR signal to a signal from one or more standards.
Typically, multiple scans for a given sample are acquired and co-averaged to improve signal to noise. The delay between acquired scans is known as the relaxation delay time and is on the order of several seconds. Ideally, one would acquire the first data point following the RF pulse from the H-NMR signal. However, it is typical to acquire data over a small sampling window, known as the data acquisition window.
U.S. Pat. No. 4,701,705 describes a low field H-NMR apparatus for conducting NMR moisture measurements. In the apparatus, a NMR apparatus 15 cooperates with a static pipe 12 or other belt or conveyer system. A flowing material passes along the static pipe. A pulse is transmitted to the coil from the NMR apparatus 15 and an output is formed which is the transient NMR response. The output signal is applied to a peak signal detector 20. The peak signal detector and the output signal are both input to a CPU 22. The output signal is first passed through a digitizer 24 which converts the analog signal into a series of digital words. The CPU collaborates with a memory 26, and periodically forms an output which is an indication of moisture. The indicator 28 provides data which typically is expressed in the form of percentage moisture content.
In FIG. 2 of U.S. Pat. No. 4,701,705, the ordinate is the transient NMR response measured in volts. Several curves extend through about 50 microseconds. A peak first occurs (at about 5 to 7 microseconds on the graph) and decay is thereafter noted. U.S. Pat. No. 4,701,705 does not describe a method for minimizing inaccuracies which may result from assymmetry of the sample flowing through the pipe 12.
Some patents describe rotating a sample about an axis that is subtantially perpendicular to the direction of the magnetic field in the gap. For example, U.S. Pat. No. 5,184,078 describes the use of an O-ring to couple the test tube to a motor. The motor can then be controlled to rotate the test tube at a desired speed. However, the system described in U.S. Pat. No. 5,184,078 is used to perform high field NMR. See col. 4, 11. 47-col. 5, 1. 10.
A sample is spun during high field NMR in order to reduce the effect of the inherent inhomogeneity in the static magnetic field to which the sample is exposed. In solid state high field NMR analysis, the sample also is spun in order to reduce or eliminate the effects of the inherent anisotropies of internal magnetic interactions which are typically averaged out in liquids but contribute to severe loss of spectral resolution in solids. The data acquired during sample spinning is then Fourier transformed to produce highly resolved peaks. The higher the peak resolution, the more accurate and complete is the identification and quantification of chemical structures present in the sample.
In high field NMR, the spinning period typically is shorter than the data acquisition window. In other words, the sample undergoes many rotations over the typical data acquisition window. So, in high field NMR the spinning is done in such a way as to allow all parts of the sample to experience many different orientations within the data acquisition window in such a way that all parts of the sample experience, on average, the same local field. One thus obtains NMR spectra with optimally narrowed lineshapes.
Low field NMR is not concerned with lineshapes because it does not involve a frequency domain spectrum and does not attempt to resolve spectral features. Methods are needed to improving the precision of hydrogen content determination when using time domain low field H-NMR analysis.